Clarify the effect of fracture propagation on force chains evolution of plugging zone in deep fractured tight gas reservoir based on photoelastic experiment

Abstract

The mesoscale structural stability of plugging zone formed by lost circulation materials (LCMs) frequently dictates the effectiveness of wellbore strengthening operations in deep fractured reservoirs. To address the stability of these mesoscale structures, researchers have conducted extensive investigations encompassing the establishment of photoelastic experimental and coupled computational fluid dynamics with discrete element simulation methods, as well as the influence of LCMs' geometric shape and frictional properties. However, contemporary studies primarily focus on the overall structural evolution of plugging zone under static fracture tip boundaries, lacking a detailed characterization of the pressure-bearing evolution process of force chain structures within plugging zone fracture tip regions under dynamic fracture tip conditions. This study employs the photoelastic experimental technique to explore the development of force chains under shear in a photoelastic granular system with a dynamic fracture confining boundary. Additionally, it utilizes the average intensity gradient method to accurately describe the evolution of average contact force, the proportion of strong force chains, and the strong force chains geometric structure in each segment under a dynamic fracture confining boundary. The test results showed that the presence of a dynamic fracture boundary leads to both a reduction in the abundance of strong force chains proximal to the fracture tip and a simplification of their geometric architecture, subsequently impacting the stability of the plugging zone. Within the low-propagation fracture plugging zone, an escalation in the shear displacement is observed uniformly progressing with an increase in shear pressure, whereas the high-propagation fracture plugging zone demonstrates climactic fluctuations in shear pressure as shear displacement intensifies. The peak shear strength of force chains within the low-propagation fracture reaches 20 MPa, a value 2.2 times higher than that of the high-propagation fracture boundary plugging zone. The ratio of strong force chains within low-propagation fracture exhibits a consistent increment from 10.5% to 13.3% in response to the ascension of shear displacement. On achieving the maximum shear displacement, this ratio in the front-middle-rear segmentations of the plugging zone reaches 14.6%, 14.7%, and 10.5%, respectively. Notably, within the high-propagation fracture, the proportion displays a fluctuating decline from 11.2% to 7.7%. At the pinnacle of shear displacement, the proportion of strong force chains in the front, middle, and rear divisions of the plugging zone are 7.6%, 7.8% and 7.7% respectively. Adjacent to the low-propagation fracture, strong force chains intermittently gather around the shear zone within the plugging area. Here, they gradually configure into staggered arrangements that eventually fuse into a densely packed, high-luminosity network resembling the structure of a fishbone. Conversely, within the high-propagation fracture regions, strong force chains transition from isolated to offset formations, leading to a persistently disrupted sparsely chain-like structure. This study provides theoretical basis for the wellbore strengthening in fracture propagation deep fractured tight gas reservoir.